The invention relates to a method of generating a carrier wave of common channels by adjusting transmission powers of time slots and by using frequency hopping in a radio system, the radio system comprising a network part, at least one subscriber terminal and a bidirectional transmission path between the network part and the subscriber terminal, and the network part comprising at least one base station, and the bidirectional transmission path consisting of frames to be transmitted on the carrier wave frequency, the frame comprising time slots to which time slot a channel is applied, the channel being either a common channel or a dedicated channel, and the method comprising the steps of transmitting at least one time slot containing a common channel with a predetermined reference power without frequency hopping, transmitting other time slots with a transmission power lower than the predetermined reference power, using frequency hopping.
In radio systems each base station has one carrier wave that is called a carrier wave of common channels. The name control channel can also be used. In fact, this term refers to a plurality of channels which a subscriber terminal and a base station need for the control of their own operation. In addition to these, on the same carrier wave frequency are transmitted the proper dedicated traffic channels, along which user payload, such as speech or data, is transmitted after the establishment of a connection. Within dedicated channels, too, signalling information can be transferred, if necessary.
The term channel is used in two senses that differ from one another. In the present description, a channel means a logical channel which contains some characteristic information. In the other sense a channel means a physical channel which is used for transferring the logical channel. In the present description, the term time slot is used for the physical channel.
In the GSM system one physical channel is one time slot of a TDMA (Time Division Multiple Access) frame. Logical channels are common channels or dedicated channels. A TDMA frame has eight time slots. A 26-multiframe, measuring 26 TDMA frames, is defined for dedicated channels, in this case the time slots 1 to 7 of the TDMA frame are used. Correspondingly, a 51-multiframe, measuring 51 TDMA time frames, is defined for common channels, in this case the time slot 0 of the TDMA frame is used.
Common channels comprise BCH channels (Broadcast Channel) and CCCH channels (Common Control Channel). BCH channels comprise FCCH channel (Frequency Correction Channel), SCH channel (Synchronization Channel) and BCCH channels (Broadcast Control Channel). CCCH channels comprise PCH channel (Paging Channel), AGCH channel (Access Grant Channel) and RACH channels (Random Access Channel). Dedicated channels comprise Traffic Channels and DCH channels (Dedicated Control Channel). DCH channels comprise SDCCH channel (Stand-alone Dedicated Control Channel), SACCH channel (Slow Associated Control Channel) and FACCH (Fast Associated Control Channel).
The common channels are relevant to the present invention. In the downlink direction are FCCH channel, SCH channel, BCCH channels and CCCH channels (PCH channel and AGCH channel). The FCCH channel comprises frequency correction information for a subscriber terminal. The SCH channel comprises frame synchronizing information. The BCCH channel comprises general basestation-specific information. Of the CCCH channels the PCH channels comprises call information and the AGCH channel comprises information on traffic channel allocation to a subscriber terminal. In the uplink direction is a RACH channel. The RACH channel comprises a request of a subscriber terminal to employ an SDCCH channel, in order that a subscriber terminal could, for instance, reply to a call or establish a call.
Frequency hopping was developed for military radio systems to make unauthorized reception difficult and to prevent interference. In interference prevention the important thing is that frequency hopping provides frequency diversity and interferer diversity. Frequency hopping is implemented in such a way that a transmitter sends a specific amount of information on a specific frequency. Then the transmitter continues sending radio information on some other frequency. This frequency change can be performed several times. In general, the frequency change is repeated as a known sequence, and this is called a frequency hopping sequence.
The carrier wave of common channels is transmitted with full power and at the same frequency, which means that frequency hopping cannot be utilized with it. This causes problems in the planning and use of a radio system.
A major problem is that, in a sense, network planning needs to be duplicated. Firstly is planned the use of normal carrier waves, which use can be enhanced by frequency hopping, discontinuous transmission and power control. Secondly is planned the use of the carrier waves of common channels.
Another major problem arises from the fact that the capacity of the system decreases when dedicated channels on the same carrier wave with common channels cannot utilize frequency hopping, discontinuous transmission and power control.
The aim of the present invention is to provide a method of generating a carrier wave of common channels, by which carrier wave the described problems can be avoided.
This is achieved with a method set forth in the preamble, which is characterized in that the method further comprises the step of transmitting at least one other time slot, determined by the transmission power sequence, with a predetermined reference power by using frequency hopping.
The invention further relates to a system for generating a carrier wave of common channels by adjusting the transmission powers of time slots and by using frequency hopping in a radio system, the radio system comprising a network part, at least one subscriber terminal and a bidirectional transmission path between the network part and the subscriber terminal, and the network part comprising at least one base station, and the bidirectional transmission path consisting of frames to be transmitted on the carrier wave frequency, the frame comprising time slots, and to which time slot a channel is applied, the channel being either a common channel or a dedicated channel, and the network part is arranged to transmit at least one time slot, containing a common channel, with a predetermined reference power without frequency hopping, to transmit other time slots with a transmission power lower than the predetermined reference power by using frequency hopping.
The system is characterized in that the network part is arranged to transmit at least one other time slot, determined by the transmission power sequence, with the predetermined reference power by using frequency hopping.
Several significant advantages are achieved with the method of the invention. The capacity of the dedicated channels on the carrier wave of the common channels improves significantly, since frequency hopping can be used. For common channels power control and discontinuous transmission can be used. Network planning becomes much easier, since the common frequency planning can be done for all carrier waves. The power measuring of a neighbouring base station, performed by a subscriber terminal, becomes more secure, since it can be performed during at least two different time slots. A very significant advantage is achieved, by the fact that the method of the invention is applicable with slight changes to the present GSM system, in case the power of the carrier wave is not reduced too much, i.e. less than about 10 dB.
The system according to the invention has the same advantages as those described above in relation to the method. The preferred embodiments of the invention and other more detailed embodiments enhance the advantages of the invention. It is obvious that the preferred embodiments and the detailed embodiments can be combined with one another to provide various combinations in order to achieve the desired technical efficiency.